The present invention relates to a variable-valve-actuation (VVA) apparatus for an internal combustion engine, which can vary at least the operating angle of engine valves such as an intake valve and an exhaust valve in accordance with engine operating conditions.
Typically, the VVA apparatus applied to intake valves comprises a crank cam arranged at the outer periphery of a driving shaft that rotates in synchronism with a crankshaft and having an axis eccentric to an axis of the driving shaft, and a valve operating (VO) cam to which torque of the crank cam is transmitted through a transmission mechanism to have a cam face coming in slide contact with the top face of a valve lifter arranged at the upper end of the intake valve for opening operation thereof against a biasing force of a valve spring.
The transmission mechanism includes a rocker arm disposed above the VO cam and swingably supported to a control shaft, a crank arm having an annular first end engaged on the outer peripheral surface of the crank cam and a second end rotatably connected to a first arm of the rocker arm through a pin, and a link rod having a first end rotatably connected to a second arm of the rocker arm through a pin and a second end rotatably connected to a cam nose of the VO cam through a pin.
The control shaft is driven, for example, by an electric motor through a worm gear or reduction mechanism provided to a driving shaft of the motor. Fixed on the outer peripheral surface of the control shaft is a control cam having an axis eccentric to an axis of the control shaft by a predetermined amount and rotatably fitted in a support hole formed substantially in the center of the rocker arm. The control cam changes a rocking fulcrum of the rocker arm in accordance with the rotated position to change the position of contact of the cam face of the VO cam with respect to the top face of the valve lifter, carrying out variable control of the lift amount and operating angle of the intake valve.
Specifically, when the engine operating conditions are in the low-rotation range, for example, the control shaft is rotated in one direction through the motor to rotate the control cam in the same direction, moving the rocking fulcrum of the rocker arm in the direction of separating from the driving shaft. Then, a pivotal point of the rocker arm with the link rod is moved upward to draw up the cam nose of the VO cam, moving the position of contact of the VO cam with respect to the top face of the valve lifter in the direction of separating from a lift portion of the VO cam. Thus, the intake valve is controlled to minimize the lift amount and the operating angle
On the other hand, when the engine operating conditions pass from the low-rotation range to the high-rotation range, the control shaft is rotated in another direction by the motor to rotate the control cam in the same direction, moving the rocking fulcrum of the rocker arm in the direction of approaching the driving shaft. Then, the cam nose of the VO cam is pushed downward by the link rod, etc. to move the position of contact of the VO cam with respect to the top face of the valve lifter to the lift portion of the VO cam. Thus, the intake valve is controlled to increase the lift amount and the operating angle.
Therefore, outstanding engine performance such as improved fuel consumption, increased engine output, or the like can be obtained in accordance with the engine operating conditions.
With the VVA apparatus, however, referring to FIG. 8, the worm gear arranged to reduce rotation of the motor for control of the control shaft has a reduction ratio which is always constant regardless of the control conditions of the valve-lift amount and the operating angle as shown by broken line. Thus, under small valve-lift and operating-angle control which corresponds to a control area in the ordinary driving range or practical range of the vehicle, the reduction ratio is smaller, leading to greater power consumption of the motor.
Specifically, the reduction ratio obtained from the angular velocities of the driving shaft of the motor and the control shaft corresponds to a torque ratio of the motor, which is proportional to current supplied thereto. Therefore, under small valve-lift and operating-angle control, the reduction ratio is not increased and thus smaller, leading to greater torque of the motor for rotating the control shaft. This increases power consumption during ordinary driving of the vehicle, resulting in a harmful effect on fuel consumption of the internal combustion engine which also serves to drive accessories such as an alternator.
Further, if power supplied to the motor is smaller due to reduction in storage amount of a battery for supplying power to the motor, a technical problem can occur such as deterioration of the rotation-ability of the motor in the ordinary driving range of the vehicle.
Furthermore, since the reduction ratio is not decreased and thus constant during the transition from small valve lift to large valve lift, which occurs at quick acceleration of the vehicle and the like, the total number of revolutions of the motor required for this transition cannot be reduced, causing longer transition time, resulting in possible lowering of the switching responsivity from small valve lift to large valve lift.